An In-situ Technique to Measure Erosion and Deposition in Fusion Devices

Erosion or deposition of sub-micron layers of graphite or other materials can be measured by bombarding a sub-surface layer of “B or ‘Li with thermal neutrons and observing with a surface-barrier detector the energy loss of the prompt alphas or tritons produced. To demonstrate the feasibility of this technique. a (525Ok 250) A layer of boron and a (1.25 +0.05) pm layer of Li,B,O, were electron-beam evaporated onto graphite substrates and exposed to a thermal neutron flux of (8.0+ 0.5) x lo5 cm -2 s-l. The (n,(2) reactions of the l”B produce a 1.78 MeV a, a 1.48 MeV CX, and a 0.848 MeV ‘Li. The reactions of ‘Li produce a 2.73 MeV 3H and a 2.05 MeV a. Carbon coatings of (600 + 25) A, (8250 f 500) A, (2.0 f 0.2) pm, and (4.0 f 0.4) pm were placed between the active layers and a surface barrier detector in vacuuo. The thinner layers shifted the 1.48 MeV a peak by (31.7i4.5) keV and (431+43) keV respectively. The thicker layers shifted the 2.73 MeV 3H peak by (206 k 15) keV and (346k20) keV respectfully. Therefore, utilizing boron implants, 100 A to 1 pm of graphite erosion or redeposition can be determined. Utilizing lithium implants, thicknesses in the range of 1 pm to 10 pm can be determined. Theoretical energy shifts, thermal diffusion, and the feasibility of this technique as a between shot diagnostic for limiters, divertor plates, and/or first-wall armor are discussed.